1. Field of the Invention
This invention relates to the purification of silicon in general, and specifically to the purification of silicon in a liquid tin-lead solution.
2. Description of the Prior Art
Processes successfully used in the past include the reduction of silicon tetrachloride with zinc, cadmium, or hydrogen, the reduction of trichlorosilane (SiHCl.sub.3) with hydrogen, the pyrolytic decomposition of silane (SiH.sub.4); and the reduction of silicon tetraiodide and silicon tetrabromide with hydrogen. In each case, the starting materials must be carefully purified and great care exercised in the selection of storage and reaction chamber materials.
The usual sequence is to make a low-grade silicon or ferrosilicon, chlorinate the impure silicon, carefully purify the resulting halogen, and then reduce it. Huge quantities of silicon tetrachloride and trichlorosilane are produced for use in silicon manufacture so a ready source of feed stock for the semiconductor industry has been available.
Halides are most often purified by direct distillation, but occasionally other methods are used with, or in addition to the distillation. If impurities in the range of 50 to 100 ohm-centimeter are desired, pyrex or steel columns are adequate, and indeed high-silicon tetrachloride and trichlorosilane are normally shipped in steel tanks.
The first commercial process for the production of semiconductor-grade silicon used zinc to reduce silicon tetrachloride. A number of problems are associated with this method. Zinc with its high melting point poses problems. The silicon grows out in dendrites, or needles, from the walls of the container and some form of densification must be used prior to charging into a crystal puller. Even though the starting materials are carefully purified before feeding into the reactor, trace impurities still remain and appear in the deposited silicon. The zinc process has been completely supplanted by various hydrogen-reduction methods.
Silane decomposition involves the pyrolytic decomposition of silane. It has not been widely used, however, because of the difficulty of making silane and because of the hazard of its instability. Silane will ignite and explode in air and is decomposed by water containing traces of alkali.
The iodide process using the decomposition of silicon tetraiodide to form silicon is used quite often, however, because of the high cost of iodine, a recovery process is necessary. In order to obtain reasonable deposition rates, low pressures are required, so that a combination of vacuum pumps and iodine traps is required. It is because of these additional requirements that the process has thus far not proved commercially feasible.
The most used method in the manufacturing of silicon is the use of silicon tetrachloride (SiCl.sub.4) and trichlorosilane (SiHCl.sub.3). The use of trichlorosilane is favored over silicon tetrachloride because of faster deposition rates and because it is apparently easier to remove phosphorus and boron compounds from it. However, as in some of the previously mentioned manufacturing processes, complicated reactor systems must be used. Most reactors use a heated quartz tube. The quartz tube is very simple, may be resistance heated and produces predominantly dense silicon with some protuberances on the inside. However, the silicon bonds to the quartz, which must be removed by leaching in hydrofluoric acid. The loss of a quartz tube each run raises the processing costs and the etching usually induces contamination, as does the quartz tube itself.
Accordingly, an object of this invention is to provide a method of purifying silicon that requires only simple materials.
Another object of this invention is to provide a method of purifying silicon at low cost.
Another object of this invention is to provide a method of purifying silicon that requires only simple apparatus.